In connection with recent progress of multimedia communications including Internet communications, researches on wavelength division multiplexing (WDM) technologies for higher-speed and larger-capacity communications are actively promoted. One of the key optical components in establishing future WDM communication systems is an optical coupling/splitting device for coupling or splitting a plurality of lights having respective wavelengths. In view of reduction of a cost and a size of the optical coupling/splitting device and enhancement in functionality thereof, it is integrated so that the device is made of silica (glass) or polymer on a substrate, and an optical transmitter and/or receiver are/is mounted on the substrate.
Several types of optical coupling/splitting devices are known, for example, a filter-type device, a directional-coupler-type device and a Mach-Zehnder-interferometer-type device.
Regarding a type of an optical coupling/splitting device which is advantageous to reducing a size of a module thereof, a filter-type optical coupling/splitting device disclosed in the Japanese Patent Laid-open Publication No. 8-190026 (Patent Publication 1) is known. In this filter-type optical coupling/splitting device, as shown in FIG. 19, two straight optical waveguides 401, 402 are intersected with each other at a junction and an optical filter 404 is embedded at the junction. This filter-type optical coupling/splitting device utilizes a property of the optical filter 404 which property allows light to be transmitted therethrough or reflected thereat depending on a wavelength of light so that a WDM light or signal can be split into a reflected light and a transmitted light. In this filter-type optical coupling/splitting device, it is necessary to design such that an intersecting point 403 between respective optical axes of the two optical waveguides 401, 402 joining together at an angle 20 is located on an equivalent reflection center plane 405 of the optical filter 404. In FIG. 19, axes of the optical waveguides 401, 402 and 430 are respectively indicated by reference numerals 406, 407 and 408.
In an above filter-type optical coupling/splitting device, reflection means such as an optical filter is mounted in a filter groove at the junction. In such optical systems or devices utilizing a reflected light produced by the reflection means, it is necessary to minimize tolerance relative to a positional deviation of the reflection means. Namely, when the reflection means is set with a positional deviation, a change in light propagating characteristics around the junction region must be as small as possible. When an input light is reflected at the reflection means, a positional deviation of the reflection means affects double an optical path length of the input light to the reflection means. Thus, in order to ensure adequate light propagating characteristics, the tolerance relative to the positional deviation of the reflection means must be as small as possible.
In order to solve a problem about the serious adverse effect on light transmitting efficiency due to such a positional deviation of the reflection means of the filter-type optical coupling/splitting device disclosed in the Patent Publication 1, the Japanese Patent Laid-open Publication No. 2002-6155 (Patent Publication 3) provides an optical coupling/splitting device which comprises first, second and third optical waveguides, a fourth optical waveguide capable of propagating light with plural propagation modes, and an optical filter disposed across a light-traveling direction in the fourth optical waveguide. The first optical waveguide is connected to a first end face of the fourth optical waveguide, and the second and third optical waveguides are connected to a second end face thereof on a side opposite to that of the first end face at respective locations. Each of the first and second end faces of the fourth optical waveguide is located across the light-traveling direction in the fourth optical waveguide. The fourth optical waveguide is operable to propagate light in a multi-mode in such a manner that light having a first wavelength input from one of the second and third optical waveguides is transmitted through the optical filter to the first optical waveguide as light corresponding to the input light having the first wavelength, and that light having a second wavelength input from one of the second and third optical waveguides is reflected at the optical filter to the other thereof as light corresponding to the input light having the second wavelength.
More specifically, as shown in FIGS. 20 and 21, in the optical coupling/splitting device disclosed in the Patent Publication 3, each of the optical waveguides is formed on a given substrate 520, such as a silicon (Si) substrate 520, using two types of fluorinated polyimide resins different in refractive index from each other. This optical waveguide comprises a first cladding layer 521, a core 522 and a second cladding layer 523. In FIGS. 20 and 21, the reference numeral 524 indicates a filter-insertion groove. Just as an example, thicknesses of the lower cladding layer 521, the core 522 and the upper cladding layer 523 are respectively 5 μm, 6.5 μm and 15 μm. A value of relative index difference between the core and the cladding layers is 0.3%.
As shown in FIG. 21, an optical coupling/splitting section includes a multi-mode interference-type optical waveguide 510, a first optical waveguide 511, a second optical waveguide 521 and a third optical waveguide 513. The multi-mode interference-type optical waveguide 510 has a width W of 25 μm and a length L of 1200 μm. A distance between the optical waveguides 512, 513 is 5 μm and each of the optical waveguides 511, 512, 513 has a width D of 6.5 μm.
The optical filter 515 of a dielectric multilayer-film type filter 515 adapted to reflect light having a wavelength of 1.31 μm and transmit light having a wavelength of 1.55 μm when the light is input into the filter 515 at an incident angle of zero degree. The dielectric multilayer-film filter 515 has a thickness of 15 μm and a conventional configuration. The dielectric multilayer-film filter 515 is inserted into a groove 524 which is formed to have a width of 15 μm and located in a central region of the multi-mode interference-type optical waveguide 510, and glued by using UV (Ultra-Violet) (not shown). The groove 524 is formed, for example, by using a dicing saw. The second and third optical waveguides 512, 513 are formed to extend parallel or approximately parallel to each other at respective connecting locations 532, 533 in which they are connected to the multi-mode interference-type optical waveguide 510.
An operational principle of the optical coupling/splitting device disclosed in the Patent publication 3 is as follows. As shown in FIGS. 21 and 22, the plurality of optical waveguides, i.e., the optical waveguides 512, 513, are connected to one of the opposite end faces of the multi-mode interference-type optical waveguide 510 at respective individual positions. In the multi-mode interference-type optical waveguide 510, an intensity peak portion of light is shifted in a direction perpendicular to the light-traveling direction according to the traveling of light.
The optical filter 515 is disposed to extend perpendicular or approximately perpendicular to the light-traveling direction in which multi-mode light is propagated through the multi-mode interference-type optical waveguide 510. This makes it possible to prevent undesirable light leakage causing noise, for example, when light input from the optical waveguide 513 is transmitted to the optical filter 515.
Further, in the optical device disclosed in Patent Publication 3 in which a thin-film optical device such as an optical filter and the multi-mode interference-type optical waveguide are disposed side-by-side, an incident angle of light into the thin-film device is close to zero degree. This makes it possible to effectively eliminate a polarization dependency of the reflected light or the transmitted light in the thin-film optical devices, and reduce polarization dependent loss (PDL).
In a usual optical system having a conventional multi-mode interference optical waveguide without employing any thin-film optical device (hereinafter, referred to “MMI”), a width of a MMI section is constant, a width of an input optical waveguide equals to that of an output optical waveguide, and a positional relationship between the input and output optical waveguides is symmetrical relative to an axis of the MMI section, as described in Japanese Patent Laid-open Publication No. 2000-221345 (Patent Publication 2). This optical system utilizes a self-imaging effect in the MMI section.
Patent Publication 1: Japanese Patent Laid-Open Publication No. 8-190026
Patent Publication 2: Japanese Patent Laid-Open Publication No. 2000-221345
Patent Publication 3: Japanese Patent Laid-Open Publication No. 2002-6155
The optical system including the optical coupling/splitting device and the waveguides disclosed in the Patent Publication 3 can solve the problem about the difficulty in production process, i.e., the requirement of minimizing tolerance relative to a positional deviation of the reflection means of the optical coupling/splitting device disclosed in the Patent Publication 1, and a problem about production cost associated with the difficulty in production process. However, a size of the optical coupling/splitting device is relatively large. If the size thereof is reduced to meet practical requirements, light transmitting efficiency thereof will be significantly deteriorated, and likely to cause light leakage between the optical waveguides.
Further, the present inventors found that, when a groove is formed in an MMI section in a direction perpendicular to an optical axis thereof and an a thin-film optical device is mounted into the groove, as disclosed in the Patent Publication 3, a light wave field caused by light input at an input end is not sufficiently converged in an output end. Thus, when an optical coupling/splitting device is configured as described in the Patent Publication 3, light to be output at an output end may cause radiation loss thereat so that insertion loss of the optical device becomes worse, and light not to be output at the output end leaks thereinto so that isolation of the optical device is deteriorated.
In view of the above problems in the conventional optical coupling/splitting device, it is an object of the present invention to provide an optical system with waveguides capable of reducing a size thereof without increasing a substantial production cost as compared with the conventional optical coupling/splitting device.
It is another object of the present invention to provide an optical system with waveguides capable of ensuring high light-transmitting efficiency while reducing light leakage between optical waveguides, to achieve highly-accuracy optical communications.